le that of the oxygen. Oxygen is more soluble in water than
hydrogen, and a very little of it is also lost by being converted into
ozone and other substances. It has been found that when the necessary
corrections are made for the error due to these facts, the volume of the
hydrogen is exactly double that of the oxygen.

Fig. 19 illustrates a simpler form of apparatus, which may be used in
place of that shown in Fig. 18. A glass or porcelain dish is partially
filled with water to which has been added the proper amount of acid. Two
tubes filled with the same liquid are inverted over the electrodes. The
gases resulting from the decomposition of the water collect in the
tubes.

[Illustration: Fig. 19]

2. _Quantitative analysis._ The analysis just described is purely
qualitative and simply shows that water contains hydrogen and oxygen. It
does not prove the absence of other elements; indeed it does not prove
that the hydrogen and oxygen are present in the proportion in which they
are liberated by the electric current. The method may be made
quantitative, however, by weighing the water decomposed and also the
hydrogen and oxygen obtained in its decomposition. If the combined
weights of the hydrogen and oxygen exactly equal the weight of the water
decomposed, then it would be proved that the water consists of hydrogen
and oxygen in the proportion in which they are liberated by the electric
current. This experiment is difficult to carry out, however, so that the
more accurate methods based on synthesis are used.

~Methods based on synthesis.~ Two steps are necessary to ascertain the
exact composition of water by synthesis: (1) to show by qualitative
synthesis that water is formed by the union of oxygen with hydrogen; (2)
to determine by quantitative synthesis in what proportion the two
elements unite to form water. The fact that water is formed by the
combination of oxygen with hydrogen was proved in the preceding chapter.
The quantitative synthesis may be made as follows:

[Illustration: Fig. 20]

The combination of the two gases is brought about in a tube called a
eudiometer. This is a graduated tube about 60 cm. long and 2 cm. wide,
closed at one end (Fig. 20). Near the closed end two platinum wires are
fused through the glass, the ends of the wires within the tube being
separated by a space of 2 mm or 3 mm. The tube is entirely filled with
mercury and inverted in a vessel of the same liquid. Pure hydrogen is
passed